Update podspec

Supersedes #143.

* Deduce tag from Pod version.
* Fix `pod spec lint` warnings.
* Remove files not directly related to the static library (Benchmarking, Demos) as Pods normally do.
* Add platform requirements (#116).
Merge pull request #148 from rivera-ernesto/podspec

Usually I won't merge my own pull requests but this one is already pushed to https://github.com/CocoaPods/Specs.